1. Field of the Invention
The present invention relates generally to a broadband wireless access communication system, and in particular, to an apparatus and method for preventing handover failure that may occur of a subscriber station (SS) recognizes a sector of another cell as a sector of a current cell when a radio access station (RAS) uses a compressed downlink-map (DL-MAP).
2. Description of the Related Art
Researchers are studying the fourth-generation (4G) communication systems, which are the next-generation communication systems, to provide users with services of diverse qualities of service (QoS) at a rate of about 100 Mbps.
Particularly, current 4G communication systems have a form of a broadband wireless access communication system, such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN), with mobility and quality of service added there to, and aim to provide services at a high data transmission rate. The representative communication systems are an Institute of Electrical and Electronics Engineers (IEEE) 802.16d communication system and an IEEE 802.16e communication system. The IEEE 802.16d and the IEEE 802.16e communication systems adopt an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to provide a broadband service network on a physical channel in the wireless MAN system. The IEEE 802.16d system is a fixed subscriber station (SS) system, that is, the IEEE 802.16d system does not consider the mobility of the subscriber station but considers only a single cell architecture. The IEEE 802.16e communication system takes the mobility of the subscriber station into consideration. The subscriber station with a mobile property is referred to as a mobile station (MS) in the present invention. Hereinafter, both mobile station and subscriber station will be referred to as portable terminals. The portable terminal is defined to be all terminals that can be carried by a user.
The IEEE 802.16e communication system can acquire the optimal transmission efficiency when data is transmitted at a high transmission rate by maintaining orthogonality among multiple sub-carriers. Also, the system is highly efficient in its use of frequencies and it is strong against multi-path fading.
Also, the OFDM/OFDMA method uses a method of forming sub-channels by using sub-carriers, which are sharable resources, to increase a channel utility among multiple portable terminals and a base station in a cell, and allocating the sub-carriers to the portable terminals based on the sub-channels.
Non-limiting examples of the IEEE 802.16d system and the IEEE 802.16e system are World Interoperability for Microwave Access (Wimax) and Wireless Broadband (Wibro) or Mobile Wimax. Portable terminals with the Wibro technology are generally laptops with a Personal Computer Memory Card International Association (PCMCIA) or personal portable terminals such as a Personal Digital Assistant (PDA).
FIG. 1 illustrates a general IEEE 802.16e communication system. Referring to FIG. 1, the IEEE 802.16e communication system has a multiple cell structure. In other words, it has a predetermined cell 100, i.e., a first cell, and another cell 150, i.e., a second cell. The IEEE 802.16e communication system includes a first base station 110 in charge of the first cell 100, a second base station 140 in charge of the second cell 150, and a plurality of portable terminals 111, 113, 130, 151, and 153. Herein, the portable terminals 111, 113, 130, 151, and 153 transmit/receive signals to/from each other based on the OFDM/OFDMA method. Among the portable terminals 111, 113, 130, 151, and 153, when a portable terminal 130 is present in a boundary area between the first cell 100 and the second cell 150, portable terminal 130 may execute a handover. In other words, when the portable terminal 130 moves toward the second cell 150 controlled by the second base station 140 while transmitting/receiving to/from the first base station 110, handover occurs and the serving base station for the portable terminal 130 is changed from the first base station 110 to the second base station 140.
In the IEEE 802.16e communication system, a portable terminal identifies a sector within a predetermined cell by using a Base Station IDentification (BS ID) on a DownLink-MAP (DL-MAP). The base station provides the portable terminal with network information by periodically transmitting a MOBile NeighBoR ADVertisement (MOB_NBR-ADV) message, in order which is a broadcast message, to identify the cell to the portable terminal.
The DL-MAP of the MOB_NBR-ADV message includes a mapping table for Medium Access Control (MAC) addresses and indexes of adjacent base stations. The mapping table is maintained for each configuration change.
The base station may acquire a 48-bit ID of an adjacent base station from the indexes of the adjacent base stations included in a MOBile SCAN REPORT (MOB_SCAN-REPORT) message or a MOBile Station HandOver REQuest (MOB_MSHO-REQ) message based on the mapping table, and process a request from the portable terminal. The MOB_SCAN-REPORT message is a message reporting a result obtained by the portable terminal scanning the surroundings, and the MOB_MSHO-REQ message is a message transmitted by the portable terminal for handover.
FIG. 2 illustrates a typical structure of a BS ID of DL-MAP information. Referring to FIG. 2, a BS ID of a typical DL-MAP includes a three-byte operator ID, which is operator information, and a 3-byte sector ID, which is sector information. Thus, a BS ID is composed of a total of 48 bits.
FIG. 3 illustrates a general DL-MAP message format. Referring to FIG. 3, there is a 48-byte BS ID in a DL-MAP message.
FIG. 4 illustrates a general compressed DL-MAP message format. Referring to FIG. 4, a compressed DL-MAP message can be identified by the content in the two most significant bits (MSB) in the first data byte. When the first two bits are 11, a message is recognized as a compressed DL-MAP message.
Whereas a general DL-MAP message is assigned with 48 bits to represent a BS ID, a compressed DL-MAP message is assigned with a one-byte operator ID and a one-byte sector ID for a BS ID.
The portable terminal can receive a compressed DL-MAP of a serving cell, and receive a compressed DL-MAP of a target cell to which the portable terminal executes a handover.
However, when only two bytes are allocated as the cell information and the sector information just as in the compressed DL-MAP message and the portable terminal is present in an area where a plurality of cells are overlapped and the sector number of a handover target cell is the same as the sector number of the serving cell although the cells are different, the portable terminal may mistake a sector of the target cell as the sector of the current serving cell due to lack of information and thus execute inter-sector handover instead of inter-cell handover. Therefore, the portable terminal can hardly execute handover successfully.